There is a demand in the oil and gas industry to improve the hit rate of locating recoverable reserves, and for increasing the percentage of oil and gas recovered from reservoirs. This has resulted in the demand for improvements in the quality of seismic surveys and in a demand for in-reservoir fluid-imaging techniques. Both these requirements demand large numbers of sensors networked together.
Similar requirements in defense applications have been met using time-division multiplexing techniques, involving interrogating a number of hydrophone elements using a single pulse of light. The technique relies on the fact that for each hydrophone along the path part of the pulse energy will be modified by the hydrophone and reflected. This results in a series of reflected light pulses returning to a detector at different times from the separate hydrophone elements. The problem with this approach is that bandwidth is limited because of aliasing effects, which also restricts dynamic range. A further problem is that the number of elements addressable by a single source is relatively limited leading to a fairly large number of expensive electro-optic sources required in the total system.
A particular demand is for large arrays of optical hydrophones which can be interrogated simultaneously over single fiber leads, in real-time with high-dynamic range, and relatively wide bandwidth response. Such hydrophones are attractive for pumping through narrow bore conduits into oil reservoirs. Conventional coiled hydrophones are unsuited for this application because their diameter is not small enough. The hydrophone needs to have a diameter of no more than around 2 mm. The implication here is that the length of fiber which can be used in each hydrophone can be no longer than around 1 m to 10 m, which is significantly shorter than the 30 m to 300 m used in conventional optical fiber hydrophone systems which are coiled. These short lengths pose significant problems for the time-division multiplexing systems currently employed. In particular, the bandwidth of the resulting system is restricted owing to aliasing effects, and the pulse length (which when coherent light from a laser is used, conventionally corresponds to around the length of each coil) becomes excessively short making the electronic instrumentation difficult to implement.
Arrays exceeding 10,000 hydrophone elements can be envisaged in thin arrays which are extremely attractive for seismic streamers. These hydrophone elements would also need to be relatively short (around 1 m to 10 m).
Apparatus suitable for the simultaneous acquisition of high-bandwidth information in very long arrays was disclosed in a previous patent application GB2284256A. Wavelength division multiplexing was used in this apparatus such that hydrophone arrays could be interrogated with broadband light, and the information from each hydrophone returned at unique wavelengths. These wavelengths were separated and routed to different detectors. This apparatus has the drawback in that it utilizes a very large number of detectors--one per hydrophone element. Nevertheless, it is probably the only way to achieve very high bandwidth (500 kHz) interrogation of very short (1 m) hydrophones. The apparatus is probably not cost-effective for very large hydrophone arrays where the bandwidth requirement is relatively modest (100 Hz to 6 kHz).
Arrays using fiber Bragg grating pairs are particularly attractive--particularly if ways can be found to eliminate, or dramatically reduce, cross-talk between hydrophones. Such cross-talk is inherent in many architectures.
Conventional electrical seismic streamers contain hydrophones which are grouped together to reduce tow noise. Such groups are typically 12.5 m long and may contain 24 hydrophones. Optical hydrophone arrays can be constructed in a similar fashion, combining the outputs of groups of hydrophones in signal processing electronics. A more cost-effective solution is to replace each hydrophone group with a single hydrophone constructed in a linear fashion. However, this approach will not have the sensitivity of the coiled-hydrophone approach.
A problem with hydrophone arrays which has been published widely in the literature is that of polarization fading. Polarization fading is particularly problematic in linear hydrophones utilizing pairs of Bragg gratings. Many solutions to polarization fading have been published but none are truly satisfactory. The most robust solutions utilize either polarization maintaining optical fiber or polarizing optical fiber throughout the apparatus. However there are cost penalties associated with such solutions.
Similar polarization-fading problems exist in other sensing interferometers for the measurement of other parameters.